Fluoropyrimidine agents constitute the only known effective therapy for patients with gastrointestinal malignancies and our efforts have focused specifically on the investigation of the target of this therapy, namely thymidylate synthase. This enzyme is responsible for the de novo formation of thymidylate required for DNA synthesis and repair. The levels of this enzyme are a critical determinant of fluoropyrimidine sensitivity and a number of therapies specifically targeting this enzyme have met with clinical success both in the advanced and adjuvant disease settings. We have recently described that is probably a major mechanism of clinical insensitivity to fluoropyrimidine agents, namely a unique autoregulatory mechanism for the control of translational efficiency of thymidylate synthase. We are currently undertaking efforts to define at the molecular level the various factors acting both in cis- and trans- that effect the thymidylate synthase protein-messenger RNA interaction. We are also investigating the interaction of this central protein with other messenger RNAs that appear to be important in that they encode proteins essential in the cycling and apoptotic pathways of malignant cells, including c-myc and p53. Given the importance of thymidylate synthase, we have developed a sensitive monoclonal antibody based assay for quantitating its level in both fresh and fixed tissue. In patients with rectal and locally advanced breast cancers, thymidylate synthase is an important prognostic marker of overall survival that appears to be independent of other known prognostic factors. Patients with high levels of thymidylate synthase appear to derive the greatest benefit from chemotherapeutic intervention in the adjuvant setting. Thus, thymidylate synthase is an important prognostic indicator in epithelial tumors and identifies subpopulations who would derive the greatest benefit from chemotherapeutic intervention. This project is being extended through collaborations with the cooperative groups, including the North Central Cancer Treatment Group, the National Surgical Adjuvant Breast and Bowel Project, and the Eastern Cooperative Oncology Group. In addition to thymidylate synthase, we are investigating Ki-67 expression, allelic loss of 18q, p53 mutations, and the mismatch repair phenotype.